Incandescent light bulbs create light by conducting electricity through a resistive filament, heating the filament to a very high temperature to produce visible light. Incandescent lamps typically include an enclosure with a tungsten filament inside and a base connector that provides both an electrical and structural support connection. Incandescent lamps are generally inefficient and require frequent replacement, and are in the process of being replaced by more efficient types of electric light such as fluorescent lamps, high-intensity discharge lamps, and, in particular, LEDs. However, when dimming an incandescent lamp, for example by decreasing the effective voltage or current through the lamp, the lamp emits a color temperature that shifts from a color temperature having a higher value, for example, 2700° K, toward a color temperature having a lower value, for example, 1700° K.
LED technology continues to advance resulting in improved efficiencies and lower costs with LEDs found in lighting applications ranging from small pin point sources to stadium lights. An LED light may be 5-10 times more efficient than an incandescent light. An LED light source may typically produce 90-150 lumens per watt (LPW) while an incandescent light source may typically produce 10-17 LPW. However, when dimmed, the light output is lowered but the color temperature of an LED typically remains substantially the same or may even shift to a slightly higher color temperature. Consumers generally prefer that a light source perform in a manner similar to an incandescent lamp and emit a color temperature that changes from a higher to lower value during dimming.
U.S. Application No. 2013/0221861 discloses LED segments with different color temperatures connected in series and powered by a rectified AC mains voltage. Power is supplied to a low color temperature segment and a number of additional segments with higher color temperatures are turned on at different levels as the amplitude of the rectified AC voltage increases and turned off at different levels as the amplitude decreases. The color temperature change of the LED segments when dimmed resembles the color temperature change of an incandescent lamp. Control circuitry and a number of switches or current controlled devices are required to determine the amplitude of the rectified AC voltage and to switch the different LED segments as the amplitude changes.
U.S. Application No. 2012/0134148 discloses a lighting device with at least two LEDs with different color temperatures and different luminous flux gradients as a function of junction temperature. The lighting device has no active components and the LEDs are selected according to their color temperature and luminous flux output so that in combination they will show a color temperature decrease as current through the device is decreased. A negative temperature coefficient resistor may be connected in series with at least one of the LEDs to achieve the desired color temperature change. The application requires that LEDs with different color temperatures be selected for specific luminous flux outputs in order to achieve the desired color temperature characteristics during dimming.
Many of the currently available solutions use a large number of additional components, multi-channel drivers and control circuitry to provide the preferred color temperature change. It would be advantageous to provide structures and techniques for decreasing the color temperature of an LED light source during dimming that overcome these and other disadvantages of the present art.